|Please set SMTP/POP3/IMAP server to mail.yourdomain.com in email client. And be sure to check the "outgoing mail server requires authentication box" (to change the authentication setting above, go to Tools -> Accounts -> Choose the E-Mail account -> Click Properties -> Click the Servers tab -> near the bottom is the checkbox for this function), otherwise you will not be able to send mail (this requirement has been put into place as a defense against spammers).
There is also a feature in your control panel to automatically setup each email account in Microsoft Outlook.
If it is your main account, the username is your main account username and main password. If it is an extra POP account, the username is your email address (firstname.lastname@example.org) and the email account password.